Problem at Low Loads with Inverted-Loop Superheaters

Problem at Low Loads with Inverted-Loop Superheaters

In tall inverted-loop superheater of the type shown earlier, one of the concerns is that the steam-side pressure drop should be larger than the gravity head to ensure flow in the downward direction; this situation arises at low loads in circuits with downward flow when the friction loss is low and is on par with gravity loss.

Figure 3.37 shows the variation of friction and gravity loss as a function of mass flow of steam in downward and upward flow section of a superheater. When steam flows upward, both the gravity loss and friction loss are additive, and hence, the sum is always positive as shown and the curve is monotonic. In the downward flow section, at low flows, the enthalpy pickup in a semi-radiant superheater will be more than in a convective superheater, and the specific volume increases resulting in lower density of steam and lower gravity loss. As the steam flow increases, the temperature and specific volume of steam reach a steady value, and the density of steam will also reach a high value and be leveled out, while the friction loss increases as the square of the flow. The gravity loss versus mass flow curve will have

a shape as shown with two likely operating points at low loads. It shows that at higher mass flows, we do not have a problem (as far as stagnation is concerned, but overheating due to low steam velocity or direct furnace radiation is another issue) as we get into the region where the friction loss overtakes the gravity loss and be in the positive slope region. However, at low loads, it is showing an ambiguous trend where for a given pressure drop, there are two possible flows, which is an unstable characteristic; any small nonuniformity in gas-side or steam-side flows can upset the flows in a given tube and place the tube in a vulnerable spot resulting in overheating due to stagnation or even reverse flow.

This trend may be exhibited even with a convective superheater with low steam-side pressure drop located behind a large screen section, but due to the lower gas temperature at low loads, the superheater is in a much safer region. When consultants specify a low steam-side pressure drop, they should also be concerned about the superheater design and the lowest load it can operate without possible stagnation concerns.

It may be noted that the curve of the furnace exit gas temperature versus heat input has

a small slope, and hence, even at low loads, the energy absorbed by the superheater in the

Δp

Δp Friction loss Total loss

h Total loss h

Δp m 1 m 2 Mass flow

Gravity loss

Δp

Friction loss

Gravity loss

0 Mass flow

Ambiguous region –Δp

Upward flow Downward flow

FIGURE 3.37

Variation of pressure drop with flow in downward flow section of superheater.

138 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

semi-radiant zone is significant (that is why the author insists on a large screen section ahead of the superheater and always design a convective-type superheater). Hence, the design should have ensured that at the minimum load, the steam pressure drop is high enough to overcome gravity loss; else, a horizontal tube design or some other configura- tion should have been selected for the superheater by the boiler designer.

To overcome the problem at low loads, a baffle was suggested in the inlet header as shown in Figure 3.36. The introduction of the baffle as shown increased the tube-side velocity and friction loss about eight times more; due to the higher tube-side heat transfer coefficient, the tubes were also cooler at full load. As the flow direction is now parallel-flow, the first- pass exit will see a lower steam temperature as well as low tube wall temperature, and the second-pass exit is located in a much cooler gas temperature region minimizing the exter- nal radiation concerns. The final desired steam temperature is reached in a much cooler gas temperature zone, and hence, the tubes will be running cooler than in the option without a baffle in the header. A large turndown is also now possible due to the higher pressure drop.